Rotor assembly for an electric machine having a coolant passage

ABSTRACT

A rotor assembly for an electric machine includes a core having at least one post and a cap wherein electrical windings are wound about the rotor assembly to define a pole. The rotation of the rotor and rotor pole relative to a stator generates a current supplied from the electric machine to a power consuming device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. §371(c) of prior filed, co-pending PCT application serial numberPCT/US2013/068307, filed on Nov. 4, 2013, which claims priority toInternational Application No. PCT/US2013/058424, titled “ROTOR ASSEMBLYFOR AN ELECTRIC MACHINE”, filed Sep. 6, 2013. The above-listedapplications are herein incorporated by reference.

BACKGROUND

Electric machines, such as electric motors and/or electric generators,are used in energy conversion. In the aircraft industry, it is common tofind an electric motor having a combination of motor and generatormodes, where the electric machine, in motor mode, is used to start anaircraft engine, and, depending on the mode, functions as a generator,too, to supply electrical power to the aircraft systems. Regardless ofthe mode, the machines typically include a rotor having main windingsthat are driven to rotate by a source of rotation, such as a mechanicalor electrical machine, which for some aircraft may be a gas turbineengine.

BRIEF DESCRIPTION

A rotor assembly for an electric machine includes a non-laminationstructure defining a core having at least one post at least partiallydefining a winding seat, a winding wound around the post and having atleast a portion received in the winding seat, a lamination structuredefining a cap coupled to the post and having a portion overlying thewinding to collectively define an axial extending winding slot with thecore, and a coolant passage located internally of the core and adjacentthe winding seat.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a sectional view of an electric machine capable of operationas a starter/generator and having a rotor assembly according to variousaspects described herein.

FIG. 2 is a partial perspective view of the rotor assembly of FIG. 1sectioned along line 2-2 of FIG. 1.

FIG. 3 is an enlarged portion of the area labeled III of the rotorassembly of FIG. 2.

FIG. 4 is a perspective view of a rotor core from the rotor assembly ofFIG. 2.

FIG. 5 is a partial sectional view of the rotor assembly according tovarious aspects described herein.

FIG. 6 is a partial sectional view of the rotor assembly according tovarious aspects described herein.

FIG. 7 is a partial sectional view of the rotor assembly according tovarious aspects described herein.

DETAILED DESCRIPTION

While embodiments of the innovation may be implemented in anyenvironment using synchronous electric machine or main machine, aspecific example of which is a generator. The generator is currentlycontemplated to be implemented in a jet engine environment. Embodimentsof the innovation may alternatively include a starter/generator and mayprovide turbine engine starting capabilities, wherein thestarter/generator provides the mechanical power to drive the turbineengine through a starting method. A brief summary of the contemplatedgenerator environment should aid in a more complete understanding.

FIG. 1 illustrates an electric machine assembly 10 mounted on or withina gas turbine aircraft engine. The gas turbine engine may be a turbofanengine, such as a General Electric GEnx or CF6 series engine, commonlyused in modern commercial and military aviation or it could be a varietyof other known gas turbine engines such as a turboprop or turboshaft.

The electrical machine assembly 10 comprises a first machine 12 havingan exciter rotor 14 and an exciter stator 16, and a synchronous secondmachine 18 having a main machine rotor 20 and a main machine stator 22.At least one power connection is provided on the exterior of theelectrical machine assembly 10 to provide for the transfer of electricalpower to and from the electrical machine assembly 10. Power istransmitted by this power connection, shown as an electrical power cable30, directly or indirectly, to the electrical load and may provide for athree phase with a ground reference output from the electrical machineassembly 10.

The electrical machine assembly 10 further comprises a rotatable shaft32 mechanically coupled to a source of axial rotation, which may be agas turbine engine, about an axis of rotation 34. The rotatable shaft 32is supported by spaced bearings 36. The exciter rotor 14 and mainmachine rotor 20 are mounted to the rotatable shaft 32 for rotationrelative to the stators 16, 22, which are rotationally fixed within theelectrical machine assembly 10. The stators 16, 22 may be mounted to anysuitable part of a housing portion of the electrical machine assembly10. The rotatable shaft 32 is configured such that mechanical force froma running turbine engine provides rotation to the shaft 32.Alternatively, in the example of a starter/generator, rotation of therotatable shaft 32 of the electrical machine assembly 10 during astarting mode produces a mechanical force that is transferred throughthe shaft 32 to provide rotation to the turbine engine.

The rotatable shaft 32 may further include a central coolant passage 28extending axially along the interior of the shaft 32. The centralcoolant passage 28 allows coolant, for example, oil or air, to flowthrough the interior of the rotatable shaft 32. In the illustratedembodiment, the second machine 18 is located in the rear of the electricmachine assembly 10 and the first machine 12 is positioned in the frontof the electric machine assembly 10. Other positions of the firstmachine 12 and the second machine 18 are envisioned.

FIG. 2 illustrates a perspective view of the main machine rotor assembly40 with at least a portion of the axial front end of the assembly 40 cutaway. The rotor assembly 40 is shown comprising a core 42 having atleast one post 44 extending radially from the core 42, about which arotor winding 46, which may be electrically isolated from each other,may be wound to define a pole 48 for the assembly 40. As shown, each offour poles 48 of the rotor assembly 40 includes one rotor winding 46,wound axially about a post 44 to define a rotor winding set 50. The core42 may be, for instance, molded, formed, or bored from a non-laminatedor non-lamination, solid or unitary body material. One such example of acore body material may be steel. Alternate body materials and formationsof the core 42 are envisioned, for instance, using additivemanufacturing.

Each pole 48 of the rotor assembly 40 further comprises a cap 52. Therotor assembly 40 further comprises additional radial elements 54 spacedradially about the assembly 40, adjacent to, and in an alternatingarrangement with, the multitude of caps 52. Each cap 52 at leastpartially overlies each post 44, pole 48, and rotor winding set 50, andis spaced from each adjacent cap 52 by the radial element 54, such thatthe collective cap 52, radial element 54, and posts 44 of the core 42 atleast partially define an axially extending winding slot 56 forreceiving the rotor windings 46.

Each cap 52 may be formed or comprised by a plurality of laminations,for instance, cobalt laminations. In this instance, cobalt laminationsmay comprise the cap 52 due to its high magnetic and electricalresistance properties, and thus, its ability to minimize eddy currentsat the surface of each pole 48. Cobalt laminations are merely oneexample of a material used to construct the cap 52, and alternatematerial composition or compositions are envisioned. Comparing the cap52 to the core 46, the cap 52 may be less electrically conductive andless thermally conductive than the core 42.

Each cap 52 is removably coupled with the posts 44 of the core 42 via aninterlocking of the cap 52 with the posts 44. As shown, the interlockcomprises a projection 58 on the cap 52 and a recess 60 on the post 44,wherein both the cap projection 58 and post recess 60 have partiallycircular, complementary cross sections, such that the projection 58 isreceived within the post recess 60 to removably couple the cap 52 to thepost 44. Alternatively, embodiments are envisioned wherein theinterlocking elements are reversed, for example, a projection 58 on thepost 44, and a recess 60 on the cap 52.

The core 42 further at least partially defines internal coolant passages62 on the posts 44, located adjacent to, and extending axially inparallel with, the winding slots 56, and radial coolant passages 64extending radially from the center of the core 42 to each internalcoolant passage 62. The internal coolant passages 62 may be, forexample, molded, formed, or bored into the core 42, and are at leastpartially separated from the rotor windings 46 by, for instance, a thinportion of the post 44, allowing for thermal transfer between thewindings 46 and the coolant. The rotatable shaft 32 may additionallyinclude a plurality of coolant passage holes 66 that are radially spacedabout the shaft 32 such that they align with radial coolant passages 64,and allows for coolant to flow from the central coolant passage 28 toand from the radial coolant passages 64.

The assembled rotatable shaft 32 with coolant passage holes 66 andcentral coolant passage 28, and core 42 with internal coolant passages62 and radial coolant passages 64 defines a coolant path wherein coolantmay fluidly traverse, flow, or be forcibly pumped from the coolantpassage holes 66, through the radial coolant passage 64, to the internalcoolant passage 62, and returned to the central coolant passage 28. Therear axial end of the rotor assembly 40 may comprise a duplicate set ofcoolant passage holes 66 and radial coolant passages 64 such that thecoolant may traverse, flow, or be forcibly pumped axially along thecentral coolant passage 28 and internal coolant passages 62 to form acoolant loop. In this example, the entire coolant loop may be internalto the core 42. Alternative flows, paths, and loops of the coolantthrough the coolant passage holes 66, radial coolant passages 64, andinternal coolant passages 62, and central coolant passage 28 areenvisioned.

Turning now to FIG. 3, the rotor assembly 40 is shown further comprisinga second projection 68 on at least a portion of the post 44 that iskeyed to be received in a second recess 70 of the radial element 54. Thesecond projection 68 and second recess 70 have similar cross sectionssuch that the projection 68 received within the recess 70 to removablycouple the radial element 54 to the core 42. Additionally, the radialelement 54 is shown further abutting a biasing element, such as a wedge72, configured such that the coupling of the radial element 54 to thecore 42 biases or secures the rotor windings 46 into the winding slot56. The biasing of the rotor windings 46 into the winding slot 56ensures a physical contact between the windings 46 and slot 56, whichserves to enhance the thermal transfer via conduction. Alternativecouplings are envisioned wherein removably coupling the compressiveradial 54 to the core 42 biases the rotor windings 46 into the windingslot 56.

The core 42 further comprises a winding seat 74 at the interface of therotor windings 46 and the posts 44, for receiving the rotor windings 46.The winding seat 74 may further comprise a thermally conductive,electrically isolating layer 76 separating the rotor windings 46 fromthe posts 44. This thermally conductive layer 76 may be, for example,formed by a coating applied to the winding seat 74. Alternativethermally conductive layer 76 formations and assemblies are envisioned,such as adhesion by glue, mechanical fastening, etc. Also as shown, theradial coolant passage 64 may further extend along a channel 78 of thewinding seat 74, wherein the channel 78 is not adjacent to the internalcoolant passage 62. Alternatively, the channel 78 may further compriseadditional internal coolant passages 62 that run axially along the axisof rotation 34, parallel to, or intersecting with the existing passages62.

FIG. 4 illustrates one embodiment of the core 42 of the rotor assembly40 with the cap 52, radial elements 54, rotor windings 46, and otherremovable components detached.

During generating operation, the rotor assembly 40 is rotated about theaxis of rotation 34 by a mechanical force, such as a turbine engine,coupled with the rotatable shaft 32. During rotation, the rotor windings46 are energized to create a pole 48, for example, DC power from arectified AC power output of the exciter rotor 14. The rotation of thepole 48 relative to the main machine stator 22 generates a power output,such as an AC power output, which is then transmitted by the electricalpower cable 30 to an electrical system, for instance, a powerdistribution node.

The DC current transmitted through the energized rotor windings 46generates heat in the windings 46. Since the core 42 is more thermallyconductive than the cap 52, a portion of the generated heat istransferred away from the rotor winding 46 via the thermally conductivelayer 76 of the winding seat 74, to the core 42. Additionally, thewedges 72 bias the rotor windings 46 toward the winding seat 74 toensure a firm thermal conduction interface between the windings 46 andthe seat 74.

The rotor assembly 40 is also configured to remove heat generated in therotor windings 46, as well as heat transferred to the core 42, via theabove described coolant paths and loops 28, 62, 64, 66, 78. Forinstance, the coolant traversing through the rotor assembly 40 maydirectly remove the heat generated by the rotor windings 46 via thethermally conductive layer 76 directly adjacent to the internal coolantpassages 62. In another instance, the heat generated may be firsttransferred to the core 42 as described above, and then transferred tothe coolant via the coolant paths and loops 28, 62, 64, 66, 78.

As the rotor assembly 40 rotates at the anticipated high rotations perminute (RPMs), the centrifugal forces tend to push the rotor windings 46radially outward, which may create a gap between the thermallyconductive layer 76 and the windings 46. This thermal transfer byconvection across the gap between the rotor windings 46 and thethermally conductive layer 76 is less effective, and thus, undesirable.The collective coupling of the cap 52, the radial element 54, and thewedges 74 to the rotor assembly 40 tend to oppose the centrifugal forceson the rotor windings 46, and help improve the thermal transfer from thewindings 46 to the coolant via conduction, by ensuring the winding 46stays in place and in contact with the thermally conductive layer 76 ofthe winding seat 74.

Additionally, during generating operation, the rotation of the rotorassembly 40 relative to the main machine stator 22 typically causes eddycurrent losses due to the changing magnetic field and/or magnetic fluxharmonics in the air gap between the energized poles 48 and stator 22.Since these eddy current losses occur mainly at or near the pole 48surface, the losses may be minimized due to the lamination structure ofthe cap 52, which is less electrically conductive, and thus lessmagnetically affected, by the losses. Fewer eddy current losses alsoresults in less heat generated by the losses at or near the pole 48surface.

During generating operation, the projection 58 and recess 60 alsoprovides a secured interlocking of the cap 52 to the posts 44 and thecore 42. For instance, during rotation, centrifugal forces may attemptto separate the cap 52 from the posts 44 and core 42. The interlockingof the cap 52 to the posts 44 and the core 42 by the interlockedprojection 58 and the recess 60 prevents or retards this separation.Also, the interlocking of the cap 52 to the posts 44 and the core 42provides an anti-rotation lock configured to retard the relativerotation of the cap 52 and the post 44. The assembly of the radialelements 54 further support the anti-rotation lock configuration.

FIG. 5 illustrates an alternative rotor assembly 140 according to asecond embodiment of the innovation. The second embodiment is similar tothe first embodiment; therefore, like parts will be identified with likenumerals increased by 100, with it being understood that the descriptionof the like parts of the first embodiment applies to the secondembodiment, unless otherwise noted. A difference between the firstembodiment and the second embodiment is that the cap 152 alternativelycomprises at least a partial trapezoidal projection 158 cross section.Correspondingly, the core 142 will have a complementary trapezoidalcross section in the recess 160 of the post 144 for removably couplingthe cap 152 to the core 142. Alternate trapezoidal, or otherwiseinterlocking cross sections, are envisioned

FIG. 6 illustrates an alternative rotor assembly 240 according to athird embodiment of the innovation. The third embodiment is similar tothe first and second embodiments; therefore, like parts will beidentified with like numerals increased by 200, with it being understoodthat the descriptions of the like parts of the first and secondembodiments apply to the third embodiment, unless otherwise noted. Adifference between the third embodiment and the first and secondembodiments is that the cap 252 further comprises at least a secondprojection, shown as a corner finger 290 spaced from and on each side ofthe projection 158, extending from the cap 252, and abutting theopposing corners of the post 244 of the core 242. As illustrated, thecorner fingers 290 have at least a partial trapezoidal cross section,but may have alternative cross sections for removably coupling to and/orinterlocking with the core 242. Correspondingly, the core 242 will havea complementary cross section in at least a second recess, such as acorner finger channel 292 of the post 244.

Alternatively, embodiments are envisioned wherein the interlockingelements are reversed, for example, corner fingers 290 on the post 244,and a corner finger channels 292 on the cap 252. Additionally, while twocorner fingers 290 and corresponding corner finger channels 292 areshown, alternative numbers of corner fingers 290 and correspondingcorner finger channels 292 are envisioned.

The corner fingers 290 may further provide secured interlocking of thecap 252 to the core 242 during generating operation. During rotation,centrifugal forces may attempt to separate the cap 252 ends, farthestfrom the pole 48, from the posts 244 and core 242. The additionalinterlocking of the cap 252 to the core 242 by the corner fingers 290and corresponding corner finger channels 292 further secures the cap 252ends, preventing separation. Also, the additional interlocking of thecap 252 to the core 242 by the corner fingers 290 and correspondingcorner finger channels 292 provides an additional anti-rotation lockconfigured to retard the relative rotation of the cap 252 and post 244.

FIG. 7 illustrates an alternative rotor assembly 340 according to afourth embodiment of the innovation. The fourth embodiment is similar tothe first, second, and third embodiments; therefore, like parts will beidentified with like numerals increased by 300, with it being understoodthat the descriptions of the like parts of the first, second, and thirdembodiments apply to the fourth embodiment, unless otherwise noted. Adifference between the fourth embodiment and the first, second, andthird embodiments is that the corner fingers 390 of the cap 352 arespaced from and on each side of the projection 158, extending from cap352, but not abutting the opposing corners of the post 344 of the core342. As illustrated, the corner fingers 390 have at least a partialtrapezoidal cross section, but may have alternative cross sections forremovably coupling to and/or interlocking with the core 342.Correspondingly, the corner finger channel 392 of the post 344 will havea complementary cross section. Alternatively, embodiments are envisionedwherein the interlocking elements are reversed, for example, cornerfingers 390 on the post 344, and a corner finger channels 392 on the cap352. Additionally, while two corner fingers 390 and corresponding cornerfinger channels 392 are shown, alternative numbers of corner fingers 390and corresponding corner finger channels 392 are envisioned.

Many other possible embodiments and configurations in addition to thatshown in the above figures are contemplated by the present disclosure.For example, one embodiment of the innovation contemplates more or fewerof the electrical machine assembly 10 components mentioned, such aspoles 48, caps 52, rotor windings 46, etc. Another embodiment of theinnovation contemplates using wedges 72 configured on different sides ofthe rotor windings 46 to bias the windings 46 into the winding seats 74.Alternatively, additional wedges 72 may be included to bias more thanone side of the rotor windings 46 into the winding seats 74. Anotherembodiment of the innovation contemplates additional coolant passageholes 66 and radial coolant passages 64 spaced axially along therotatable shaft 32, such that additional coolant paths and/or loops maybe utilized to improve cooling. Additionally, the design and placementof the various components may be rearranged such that a number ofdifferent in-line configurations could be realized.

In yet another embodiment of the innovation, the rotor windings set 50and winding seats 74 may be configured with, for instance a 10°clockwise and counter-clockwise rotation, compared to the illustratedexample. It is envisioned that, for instance, the rotor windings 46 onone side of the pole 44 may be configured in a counter-clockwiseorientation, while the corresponding windings 46 on the opposing side ofthe pole may be configured in a clockwise orientation. In this example,each rotated rotor windings 46 may be held in place by one or morewedges 72, to ensure thermal contact between the windings 46 and thewinding seat 74. Also, additional components such as the cap 52 andthermally conductive layer 76 may also include slightly angled surfacesto match the rotated rotor windings 46. A 10° counter-clockwise rotationis one example of a configuration, and other angles are envisioned inboth a clockwise and counter-clockwise direction.

The embodiments disclosed herein provide a generator rotor with a capand core assembly. One advantage that may be realized in the aboveembodiments is that the above described embodiments have significantlyimproved thermal conduction to remove heat from the assembly. Theimproved thermal conductivity of the core, compared to a core comprisinglaminated materials, coupled with the coolant paths and/or loops providefor heat removal in a much more effective fashion from the windings tothe coolant. Another advantage of the above embodiments is that thethermally conductive layer replaces the typical slot liner in the rotorin a way to provide improved mechanical integrity, along with improvedthermal conductivity. The thermally conductive layer also replacesthermal conduction or cooling tubes and/or fins, reducing the number ofparts, and thus increasing the reliability of the rotor assembly. Theincreased thermal dissipation of the rotor assembly allows for a higherspeed rotation, which may otherwise generate too much heat. A higherspeed rotation may result in improved power generation or improvedgenerator efficiency without increasing generator size.

Yet another advantage of the above embodiments is that the embodimentshave significantly reduced manufacturing costs due to reduction in theamount of laminated materials, which are typically costly to produce, byreplacing the core with a non-laminated material, which is typicallyless costly to produce. Additionally, by using a solid body core, costscan further be reduced by manufacturing processes such as boring, andauto-winding of the rotor windings. Furthermore, additional coolingtubes and fins may be eliminated from construction and assembly, whereinthe prior process of welding the tubes to the assembly providedincreases costs.

When designing aircraft components, important factors to address aresize, weight, and reliability. The above described rotor assemblies havea decreased number of parts, making the complete system inherently morereliable. This results in possibly a lower weight, smaller sized,increased performance, and increased reliability system. The lowernumber of parts and reduced maintenance will lead to a lower productcosts and lower operating costs. Reduced weight and size correlate tocompetitive advantages during flight.

To the extent not already described, the different features andstructures of the various embodiments may be used in combination witheach other as desired. That one feature may not be illustrated in all ofthe embodiments is not meant to be construed that it may not be, but isdone for brevity of description. Thus, the various features of thedifferent embodiments may be mixed and matched as desired to form newembodiments, whether or not the new embodiments are expressly described.All combinations or permutations of features described herein arecovered by this disclosure.

This written description uses examples to disclose the innovation,including the best mode, and also to enable any person skilled in theart to practice the innovation, including making and using any devicesor systems and performing any incorporated methods. The patentable scopeof the innovation is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal languages of the claims.

What is claimed is:
 1. A rotor assembly for an electric machinecomprising: a non-lamination structure defining a core rotational aboutan axis of rotation and having multiple posts and at least oneprojection disposed between the multiple posts; a winding wound aroundeach of the multiple posts; a winding seat located at an interface ofthe winding and each of the multiple posts, the winding seat spaced fromthe at least one projection, having at least two surfaces and definingan electrically isolating layer separating the winding from each of themultiple posts; a lamination structure defining a cap coupled to each ofthe multiple posts, and having a portion overlying the winding tocollectively define an axial extending winding slot with the core; atleast one internal coolant passage located internally of the coreadjacent to and at least partially separated from the winding by thewinding seat, wherein one of the at least two surfaces of the windingseat is adjacent to the portion of each of the multiple posts, andextends axially in parallel with the internal coolant passage; a centralcoolant passage extending axially along the axis of rotation within thecore; and at least one radial coolant passage extending outward from thecentral coolant passage to fluidly couple the at least one coolantpassage to the central coolant passage; wherein adjacent caps coupled toeach of the multiple posts are spaced from each other by a radialelement coupled to the at least one projection.
 2. The rotor assembly ofclaim 1 wherein the central coolant passage comprises a bore in thecore.
 3. The rotor assembly of claim 1 wherein the internal coolantpassage, central coolant passage, and the at least one radial coolantpassage together form part of a coolant loop.
 4. The rotor assembly ofclaim 3 wherein at least a portion of the at least one radial coolantpassage extends along a channel adjacent another of the at least twosurfaces of the winding seat.
 5. The rotor assembly of claim 1 whereinthe core further comprises a thermal conductor provided on the windingseat.
 6. The rotor assembly of claim 5 wherein the thermal conductor isa coating applied to the winding seat.
 7. The rotor assembly of claim 1further comprising at least one wedge configured to bias the windinginto the winding seat.
 8. The rotor assembly of claim 7 wherein the atleast one wedge comprises a biasing element.
 9. The rotor assembly ofclaim 1 wherein the cap is made from material that is less electricallyconductive than the core.
 10. The rotor assembly of claim 9 wherein thecore is made from material that is thermally more conductive than thecap.
 11. The rotor assembly of claim 9 wherein in the cap is made fromcobalt and the core is made from steel.
 12. A rotor assembly for anelectric machine comprising: a non-lamination structure defining a corerotational about an axis of rotation and having multiple posts and atleast one projection disposed between the multiple posts and a centralcoolant passage extending axially along the axis of rotation; a windingwound around each of the multiple posts; a winding seat located at aninterface of the winding and each of the multiple posts, the windingseat spaced from the at least one projection, having at least twosurfaces and defining an electrically isolating layer separating thewinding from each of the multiple posts; a lamination structure defininga cap coupled to each of the multiple posts and having a portionoverlying the winding to collectively define an axial extending windingslot with the core; a channel adjacent the winding seat; and at leastone radial coolant passage extending outward from the central coolantpassage to fluidly couple the central coolant passage to the channel;wherein adjacent caps coupled to each of the multiple posts are spacedfrom each other by a radial element coupled to the at least oneprojection.
 13. The rotor assembly of claim 12 further comprising atleast one internal coolant passage located internally of the core,radially spaced from the central coolant passage, fluidly coupled withthe central coolant passage by the at least one radial coolant passage,and adjacent to and separated from the winding seat.
 14. The rotorassembly of claim 13 wherein the at least one internal coolant passage,the at least one radial coolant passage, and the central coolant passageat least partially form a coolant loop entirely internal to the core.15. The rotor assembly of claim 14 wherein the core further comprises afirst radial passage proximate to a first axial end of the core and asecond radial passage proximate to a second opposite axial end of thecore, and wherein the first and second radial passages form part of thecoolant loop.
 16. The rotor assembly of claim 15 wherein the firstradially passage is a portion of the coolant loop to deliver coolantflow from the central passage to the internal coolant passage andwherein the second radially passage is a portion of the coolant loop toreturn coolant flow from the internal coolant passage to the centralpassage.